Interspecific Competition Among Subtidal Limpets: Effect of Substratum Heterogeneity

In sublittoral barren—grounds in New South Wales, much space is occupied by crustose coralline algae, the surfaces of which are grazed by several species of invertebrates, including two limpets, Patelloida alticostata and the smaller P. mufria. The densities of these two species were negatively correlated across various habitats. More P. mufria were found in shallow (2—3 m depth) habitats where there were heterogeneous, or pitted, substrata, and in deep (>5 m) habitats with intermediate heterogeneity of the substrata, than in shallow habitats with smooth surfaces. More P. alticostata were found in shallow habitats with smooth surfaces than in Deep or shallow Pitted areas. Experimental enclosures were used to maintain limpets for 3 mo in various combinations and densities in three habitats (Smooth and Pitted shallow, and a Deep habitat with intermediate heterogeneity of the substratum), to determine how much of the observed differences in distribution and abundance of the two species was due to competitive interactions between them. In Smooth habitats, densities of P. mufria increased in enclosures without P. alticostata, but declined or remained at initial numbers where the larger limpets were present. Recruitment of juvenile P. mufria and survival of the original limpets were both greater in enclosures without than with P. alticostata, demonstrating interspecific competition. These competitive interactions were not found in Pitted and Deep habitats, and were not reciprocal; there was little mortality of P. alticostata anywhere, and no effect of P. mufria. Recruitment of P. alticostata varied among habitats, but again was unaffected by interspecific interactions. In contrast, growth, relative mass of tissue, and gonad index of P. mufria were not affected by the larger species, although mean values of the latter two variables were greater in the Smooth habitat than elsewhere. There was, however, interspecific competition from P. mufria reducing the growth, relative mass of tissues, and gonad index of P. alticostata in Deep and Pitted habitats, but not in the Smooth habitat. There was no evidence of competitive interactions from analyses of gut contents, nor from the densities and sizes of marks made on the substrata by the radulae of grazing limpets. The experiments demonstrated that competitive interactions between the two limpets affected abundances, sizes, or condition (including reproductive potential) in some, but not all, habitats studied. The mechanisms of these competitive interactions were not unambiguously identifiable, and other species (such as sea urchins) might modify the interactions. The intensity and outcome of competition between these two species varied among habitats differing in heterogeneity of the substratum and in depth. The interactions identified, and their variation among habitats, allowed interpretation of the differences in abundances and life histories of the two species from one place to another. Such experiments over a range of habitats provide more insight into the role of interspecific competition as a determinant of distribution, abundance, and life histories of closely related species than is possible from similar studies at one place or time.